Method of making an abradant element for spark generating device



Nov. 28, 1961 R. P. SEELIG EIAL METHOD OF MAKING AN ABRADANT ELEMENT FORSPARK GENERATING DEVICE Filed May 8, 1957 INVENTOR5 Ava/MP0 5592/ FIG.

Fa/7%,?) L. WAC/HELL Mir, Hm: HZM.

ATTORNEYS METHOD OF MAKING AN ABRADANT ELEMENT FOR SPARK GENERATINGDEVICE Richard P. Seelig, Hartsdale, and Richard L. Wachtell,Larchrnont, N.Y., assignors to Chromalloy Corporation, White Plains,N.Y.

Filed May 8,1957, Ser. No. 657,917 4 Claims. (Cl. 1486) This inventionrelates to mechanical spark-generating devices. Such devices commonlycomprise a ferrous metal abradant element having a roughened surface andan element made of a pyrophoric material, commonly called a flint. Theelements are rubbed against one another to cause the rough surface ofthe abradant element to remove from the pyrophoric element particles ofthe pyrophoric material that ignite spontaneously in air to form a sparkwhich can in turn be used to ignite a gaseous or liquid fuel.

A typical example of such a spark-generating device is the conventionalcigarette lighter wherein aferrous metal spark wheel having a serratedperipheral surface is positioned in contact with one end of a tubularpyrophoric element, and the pyrophoric element is held against theperiphery of the spark wheel under light spring pressure. In use thespark wheel is either manually or mechanically rotated to produce, byits abrasive action on the pyrophoric element, a spark for igniting thefuel of a fuelsaturated wick located adjacent to the spark generatingdevice.

Another example of such a spark-generating device is the gas lighterwhich at one time was widely used for igniting the gas of gas-burningdomestic cooking stoves and is still extensively used as a gas igniter.Such gas lighters comprise an abradant plate or cylinder against which aspring-mounted pyrophoric element bears in such manner that thepyrophoric element can be drawn across the abradant plate to produce thedesired gas-igniting spark.

The present invention is particularly .concerned with the abradantelement of such a spark-generating device and it is a general object ofthe invention to provide an abradant element of this character whichhasa longer useful life and greater reliability than the abradant elementsheretofore available foruse in spark-generating devices. For conveniencethe invention will be described herein as applied to the spark wheel ofa cigarette lighter, although as the description proceeds it will beapparent that the invention can equally well be embodied in abradantelements having other geometrical configurations and used for otherpurposes.

Known types of ferrous metal spark wheels with abradant teeth or ridgesformed in the periphery thereof have heretofore been made from highcarbon-steels, i.e. steels having a carbon content of 0.95 to 1.25% byweight carbon, as well as from low carbon steels with subsequent surfacehardening of the abradant surface of the wheel by a variety of knowntechniques. In spite-of these prior efforts to produce a durableproduct, the spark wheels previously available have had a relativelylimited useful life. Rotation of such a spark wheel against thepyrophoric element tends to dull, bend or otherwise deform the cuttingor abrading edges or ridges of the periphery of the wheel, thuseffectively destroying the utility of the lighter in which the wheel ismountedafter say 5,000-10,000 operations. Our studies indicate that thelimited useful life of such spark wheels is due in large measure to thefact that the materials used in their manufacture, while relatively hardat atmospheric temperatures, have not had a sufiiciently high softeningtemperature. In operation, the abradant surface of the wheel issubjected to localized heat and pressure which soften 2 the material ofthe cutting edges or-ridges, thuscausing them to be bent, blunted orotherwise deformed in such manner that they functionrinefliciently orinetfeetively.

It is accordingly an object of the present invention to provide anabradant element for a spark-generating device -having improveddurability. It is another object of the invention ,to provide anabradant element such as a spark wheelhaving an abradant surface whichis harder and retains its hardness at higher. temperatures than theabradant surfaces of abradant elements heretofore used.- It is stillanother object of the invention to provide an improved method of makingsuch an abradant element.

Other objectsof the. invention will be in part obvious andin partpointedout hereafter.

The objects of the invention can be achievedin general by formingon aspark-wheel of conventional configuration ,a hardened surface layeressentially composed of chromium carbide. It has been found that whensuch a chromium carbide layer is properly formed at the abradant surfaceof the spark wheelas described more particularly hereafter a producthavinggreatly improved durability is achieved. The atmospherictemperature hardness is relatively high, i.e. of the-order of 1800 to2200 on the Vickers scale, and nosignificant softening of the surfacematerial occurs at temperatures as high as 1800 to 2000" F. Moreoverthehardness of the material is such that even when the abradantprojections on the periphery of the wheel are overstressed, the wheelremains functionally effective. Thus if the localized pressure on aperipheral ridge or tooth of the wheel exceeds that which the ridge cansustain, the ridge does not bend or deform plasticly but rather a smallpiece breaks olf along a jagged fracture line to leave asurface that isstill abradant. It has been found that the spark wheels embodying thepresent invention have a useful life, in terms of the number of sparkgenerating operations they can perform .before becoming ineffective, ofthe order offour times that of the spark wheels previously available.

In order to point out more fully the nature of the present inventionreference will now be made to the accompanying drawing which illustratesa spark wheel embodying the present invention and an, apparatus which isuseful in carrying out the method of the invention.

In the drawings:

, FIGURE 1 is a side elevation, partly broken away, of the upper portionof a cigarette lighter illustrating the spark wheel, flint and wickassembly thereof, 4

FIGURE 2 is a right-end view of the structure of FIGURE 1,

FIGURE 3 is a greatly enlarged section through the abradant surface ofthe ,spark wheel taken on the line 3-3 of FIGURE Zand illustrating thehardened layer at the operatingsurface of the spark wheel, and

FIGURE 4 is a vertical longitudinal section through a retort adapted tobe used in carrying out certain steps of the method of the invention,

Referring to the drawing and more particularly to FIGURES 1 and 2, aportion of a conventional cigarette lighter is illustrated thereincomprising a body or fuel receptacle 10 having a spark wheel-12rotatably mounted thereon. For-med in the body 10 there is a tube 14 inwhich a rod-shaped pyrophoric element such as the conventional flint 16is slidable. The element 16 is urged by a spring 18 toward the abradantsurface 20 of spark wheel 12, -Surfaoe-20 is provided with ridges, teethor other serrations of any suitable and well-known type. Afuel-saturated wick 22 is positioned near the spark wheel. Rotationofthe spark wheel 12 in contact with element 16 generates a spark whichignites the fuelsaturated Wick in known manner.

In producing spark wheels of the type disclosed and claimed herein,spark wheel blanks having the desired ridges or other serrations in theperiphery thereof are formed from a suitable ferrous metal core whichmay be either a high carbon or low carbon steel. Since the desiredserrations at the periphery of the blank can be more readily andeconomically formed if a low carbon steel is used, we prefer, to use arelatively mild steel having a carbon content of less than say 0.7% byweight forthis purpose.

In accordance with a preferred embodiment of the method of the presentinvention, ferrous spark wheel blanks made of such a mild steel arefirst subjected to a carburizing treatment to increase substantially thecarbon content at the surface of the wheel. Carburization of the sparkwheel blanks is eifected by embedding them in a pack composed of asuitable comminuted carbnrizing medium in a container and introducingthe container into a furnace which is maintained at a temperature of1700 F. to 1900 F. for a period of 4 to hours. Any of variouscommercially available carburizing media may be used. Such carburizingmedia commonly comprise a mixture of carbon and various carbonates. Thetime and temperature may be varied according to the degree ofcarburization desired. In general the carburizing treatment ispreferably carried out in such manner that the carbon content of thesurface of the spark wheel blank is increased to say'0.8% to 3% byweight to a depth of at least M Carburization of the spark wheels mayalso be effected by gas carburization.

After carburization the spark wheel blanks are removed from thecarburizing medium and then subjected to a chromizing treatment.Chromizing is desirably effected by embedding the blanks in a suitablechromizing pack and heating them therein to an elevated temperature. Thechromizing pack may comprise for example a finely divided mixture offerrochrome and a filler such as kaolin, either calcined or uncalcined,or alumina, together with a small amount of ammonium iodide. Heating ofthe spark wheel blanks in the chromizing pack is carried out at atemperature and for a period of time sufficient to provide a hardenedchromium carbide casing at least 0.00075 thick at the surface of thewheel. The desired chromium-containing hardened casing can be achievedby using essentially the same heating schedule as in the carburizingstep, i.e. 1700 to 1900 F. for a period of 4 to 10 hours.

The nature of the hardened casing thusobtained is illustrated in FIGURE3 of the drawings which is a section through two of the teeth or ridgesof the abradant surface 20 of spark wheel 12. Referring to FIGURE 3, itwill be noted that the hardened casing is essentially bi -laminar andcomprises the outer layer 50 and inner layer 52 at the surface of theferrous metal spark wheel core 54. The outer layer 50 is relatively highin chromium content and is composed almost entirely of chrocarbidehaving substantially the formula Cr C. The inner layer 52 contains acertain amount of iron in combination with chromium and carbon andappears to be essentially a mixture of chromium carbide havingsubstantially the formula CD103 and an iron chromium carbide, probably(CIFe)qC As indicated above the outer chromium carbide layer 50 has aVickers hardness of about 1800 to 2200.

In order to illustrate further the method of the present invention the'following specific example is given of a preferred procedure for makingspark wheels embodying the invention. Referring particularly to FIGURE 4of the drawings, the spark wheel blanks 24, made of steel having lessthan 0.7% carbon, are strung on a rod or wire 26 and packed in thecomminuted carburizing medium 28, which may be the medium soldcommercially under the trade name Pear-lite, in the container 30.

After the spark wheel blanks 24 have been packed in the carburizingmedium in the container 30 a loosely fitting cover 32 is placed on topof the pack and a layer of a suitable sealing medium 34 in then spreadon top of cover 32. The sealing medium can be a finely divided silicatewhich softens to form a gas impervious seal at a temperature of theorder of 1400" F. The container 30 is placed in a tray 36 and an outercover 38 is then positioned over the container. As illustrated the sidewalls of outer cover 38 are positioned between container 30 and thewalls of tray 36. A sealing material 40 which may be the same as thesealing material 34 is positioned adjacent to the lower rim of cover 38to provide a second seal.

The assembled'tray and container are then introduced into a furnace inwhich they are heated to about 1900 F.

for about 6 hours. During the early stages of the heating cycles gasesare evolved from the carburizing material 28 that pass through theseals. 34 and 40 to the atmosphere. 'Ihese evolved gases sweep out anyatmospheric oxygen that may have been retained in the pack 28. As thetemperature rises the seals 34 and 40 melt but still permit passage ofgas therethrough. At the end of the heating cycle the tray 36 is removedfrom the furnace and allowed to cool. During cooling the seals 34 and 40solidify to prevent atmospheric oxygen from entering the pack 28 andreaching the treated spark wheel blanks during the cooling period,thereby preventing oxidation of the surfaces of the blanks during thisperiod. The average carbon content of the surface portion of the sparkwheel blanks to a depth of ,5 at the end of this carburizing treatmentis about 1.5% by weight.

7. When the carburizing treatment has been completed the spark wheelsare removed from the carburizing pack and then submitted to a chromizingtreatment. The chromizing of the spark wheel blanks may be carried outin the same type of retort as that described above, but with a difierentpack composition. The chromizing pack of the present example comprisesabout 35% by Weight of alumina, about 65% by weight of ferrochromehaving a chromium content of about 65 and about 0.25% of ammoniumiodide. All components of the pack have a particle size less than 100mesh. 'Ihe retort containing the chromizing pack with the spark wheelsembedded therein is heated at about 1880 F. for a period of 6 hours. Asthe pack is heated up the ammonium iodide decomposes to form ammonia andelemental iodine, and at a somewhat higher temperature the ammoniadecomposes to form nitrogen and hydrogen. A portion of the generatedgases flow out through the seals 34 and 40, thereby sweeping out anyatmospheric oxygen present in the pack. The hydrogen formed bydecomposition of the ammonia provides a'reducing atmosphere and reactswith any oxide coating that may be present on the spark wheels. As thetemperature of the pack continues to rise, the iodine reacts with thechromium of the pack to form a volatile iodide from which chromium isdeposited on the spark wheel surfaces. The deposited chromium diffusesinto the spark wheel blanks to form the bi-laminar hardened casingdescribed above.

At the end of the heating period the retort and pack are cooled and thespark wheels removed therefrom. The resulting spark wheels comprise aferrous metal core having a serrated abradant peripheral surfaceprovided with a bi-laminar hardened casing, the outer layer of which isa very hard coating of chromium carbide.

From the foregoing description it is apparent that the present inventionprovides an abradant element for a spark-generating device and a methodof making the same that are capable of achieving the several objects setforth at the beginning of the present specification. The exceptionallyhard chromium carbide coating substantially'increases the useful life ofthe abradant element. Moreover the nature of the casing or coating issuch that if the abradant serrations are over-stressed they break off toleave a jagged surface that is still abradant.

It is of course to be understood that the foregoing description isillustrative only and that numerous changes can be made in thematerials, conditions and proportions set forth without departing fromthe spirit of the inven tion as defined in the appended claims.

We claim:

1. The method of making an abradant element for a spark-generatingdevice which comprises forming a ferrous metal blank having a serratedabradant surface having sharp points, carburizing said abradant surfaceto increase the carbon content of the portion of said blank adjacent tosaid surface, and thereafter pack chromizing said carburized abradantsurface to form a hardened casing at said surface essentially composedof chromium carbide While maintaining the sharpness of said sharp pointsof said serrated surface.

2. The method of making a spark wheel adapted to be used in thespark-generating assembly of a cigarette lighter which comprises,forming from steel having a carbon content less than 0.7% by weight acylindrical blank having a serrated abradant cylindrical surface withthe outer edges of the serrations being sharp, carburizing said abradantsurface to increase the carbon content of the peripheral area of saidwheel to a depth of at least to an average carbon content of 0.8% to 3%by Weight, and thereafter pack chromizing said carburized surface ofsaid wheel to provide a layer of chromium carbide at the abradantperipheral surface thereof having a thickness of at least 0.00075 whilemaintaining the sharpness of said sharp edges of said serrations.

3. The method of making a spark wheel adapted to be used in thespark-generating assembly of a cigarette lighter which comprises,forming from steel having a carbon content less than 0.7% by weight aspark wheel blank having a serrated abradant peripheral surface,embedding said blank in a comminuted carburizing medium, heating theblank in said carburizing medium to a temperature of 1700 to 1900 F. fora period of 4 to hours to increase the carbon content of the peripheralportion of said blank to 0.8% to 3% by weight to a depth of at least 4removing said spark wheel blank from said carburizinlg medium andembedding it in a chromizing pack essentially composed of a mixture offinely divided ferrochrome and alumina and a small amount of ammoniumiodide, and heating said spark wheel blank in said chromizing pack to atemperature of 1700" to 1900 F. for a period of four to ten hours toform at the peripheral portion thereof a hardened bi-laminar casinghaving a thickness of at least 0.00075" and composed largely of chromiumcarbide.

4. In a method for the manufacture of an article having a serratedsurface, the steps which comprise forming from steel a blank of saidarticle having said serrated surface, carburizing said serrated surfaceof said blank for the hardening and strengthening thereof, andthereafter chromizing said carburized surfaces of said steel blank byheating said blank embedded in a chromizing pack comprising a source ofchromium and finely divided refractory filler and a source of volatilehalogen for diffusion coating of said chromium into said carburizedsurface of said blank to form an outer diffusion coated layer comprisingchromium carbide on said serrated surface of said article.

References Cited in the file of this patent UNITED STATES PATENTS1,998,496 Fiedler Apr. 23, 1935 2,048,276 Marlies et al July 21, 19362,443,123 Solon June 8, 1948 2,804,412 Anderson Aug. 27, 1957 2,804,413Essig et al. Aug. 27, 1957 OTHER REFERENCES Protective Coatings forMetals, by Burns et al., page 185, Reinhold Publishing Co., N.Y.C.,1939.

Merriman: A Dictionary of Metallurgy, 1958, published by Macdonald andEvans, Ltd., London, page 36.

1. THE METHOD OF MAKING AN ABRADANT ELEMENT FOR A SPARK-GENERATINGDEVICE WHICH COMPRISES FORMING A FERROUS METAL BLANK HAVING A SERRATEDABRADANT SURFACE HAVING SHARP POINTS, CARBURIZING SAID ABRADANT SURFACETO INCREASE THE CARBON CONTENT OF THE PORTION OF SAID BLANK ADJACENT TOSAID SURFACE, AND THEREAFTER PACK CHROMIZING SAID CARBURIZED ABRADANTSURFACE TO FORM A HARDENED CASING AT SAID SURFACE ESSENTIALLY COMPOSEDOF CHROMIUM CARBIDE WHILE MAINTAINING THE SHARPNESS OF SAID SHARP POINTSOF SAID SERRATED SURFACE.